Method for development of electrostatic images

ABSTRACT

DEVELOPER IS FED INTO A CAVITY FASTER THAN IT IS DISCHARGED CREATING A DEVELOPER ACCUMULATION IN THE CAVITY FORMING A SOFT, AGITATED DEVELOPMENT MEDIUM. IN PREFERRED EMBODIMENTS, TWO MAGNETIC BRUSHED ARE ARRANGED TO CREATE THE ACCUMULATION IN THE FORM OF A GENTLY TUMBLING ROLLBACK FROM THE CONTACTING OF ONE OF THE BRUSHES WITH THE IMAGE-BEARING SURFACE. IN THE PREFERRED EMBODIMENTS, THE ACCUMULATION IS ACCENTUATED BY ADJUSTING THE SPACING BETWEEN THE BRUSHES AND THE IMAGE-BEARING SURFACE, ADJUSTING THE PERIPHERAL SPEED OF THE BRUSHES AND CONTROLLING THE DIRECTION OF MOVEMENT OF THE IMAGE-BEARING SURFACE RELATIVE TO THE MOVEMENT OF THE BRUSHES. CARRIER CARRYOUT IS REDUCED BY DIRECTING A STRONG MAGNETIC FLUX AT THE ROLLBACK AREA AND BY PROVIDING AN AUXILIARY CARRIER SCAVENGING MECHANISM.

SH. l OF 2 Nov. 21, 1972 R DREXLER ETAL 3,103,395

METHOD FOR DEVELOPMENT OF ELECTROSTATIC IMAGES Original Filed Feb. 29,1968 42 Sheets-Sheet 1 5. FIG.|

SH. 2 OF 2 Nov. 21; 1972 R. A. DREXLER ET AL METHOD FOR DEVELOPMENT OFELECTROSTATIC IMAGES Original Filed Feb. 29, 1968 .2 Sheets-Sheet 2United States Patent M 3,703,395 METHOD FOR DEVELOPMENT OF ELECTROSTATICIMAGES Roger A. Drexler and Conrad Altmann, Rochester, N.Y., assignorsto Eastman Kodak Company, Rochester, N.Y. Original application Feb. 29,1968, Ser. No. 709,281, now Patent No. 3,543,720, dated Dec. 1, 1970.Divided and this application July 20, 1970, Ser. No. 63,961

Int. Cl. G03g 13/08, 15/08 U.S. Cl. 11717.5 1 Claim ABSTRACT OF THEDISCLOSURE Developer is fed into a cavity faster than it is dischargedcreating a developer accumulation in the cavity forming a soft, agitateddevelopment medium. In preferred embodiments, two magnetic brushes arearranged to create the accumulation in the form of a gently tumblingrollback from the contacting of one of the brushes with theimage-bearing surface. In the preferred embodiments, the accumulation isaccentuated by adjusting the spacing between the brushes and theimage-bearing surface, adjusting the peripheral speed of the brushes andcontrolling the direction of movement of the image-bearing surfacerelative to the movement of the brushes.

Carrier carryout is reduced by directing a strong magnetic flux at therollback area and by providing an auxiliary carrier scavengingmechanism.

CROSS-REFERENCES TO RELATED APPLICATIONS This is a division of US.application Ser. No. 709,281, filed Feb. 29, 1968, now US. Pat. No.3,543,720 dated Dec. 1, 1970'.

Reference is made to commonly assigned co-filed US. application Ser. No.709,280, now US. Pat. No. 3,457,- 900, entitled Single Magnetic BrushApparatus for Development of Electrostatic Images filed even dateherewith in the name of Roger A. Drexler.

BACKGROUND OF THE INVENTION This invention relates to electrostaticimages (which term is meant to include all electrostatic chargepatterns, regardless of the method of formation) and, more specifically,to methods and apparatus for developing electrostatic images carried onan insulating surface.

In electrography, it is common to form an electrostatic image on aninsulating surface and to develop that image by applying toner particlesthereto. The resulting toner image is then utilized. In the most commoncommercial applications, the toner is either transferred in imageconfiguration to another surface and then fixed or is fixed to theinsulating surface itself. In processes in which the toner istransferred from the insulating surface prior to fixing, the insulatingsurface generally is reused.

Triboelectric developing systems have been adapted to the generaldevelopment of electrostatic images. In such systems, finely dividedtoner particles are held to the surface of much larger carrier particlesby electrostatic charges created by triboelectri fication, forming amixture (herein called a developer). When the developer is brought intocontact Wtih an electrostatic image, the attraction of the image for thetriboelectrically charged toner overcomes the attraction of the carrierfor the toner and the image is developed.

Among triboelectric developing systems, the most commonly used arecascade systems and magnetic brush systems. In cascade systems, gravityis used to roll developer across the image. Because cascade systems usegravity as their primary moving force, they are neces- Patented Nov. 21,1972 sarily speed limited. In automatic machines, a cascaderecirculation system generally requires substantial machine space.

In magnetic brush systems, the carrier particles are ferromagnetic innature. These ferromagnetic carrier particles are held to an applicatorsurface, for example, a non-magnetic cylinder, in bristle formation bymagnetic attraction. With the proper use of applicator, one or moremagnets and developer, the bristles can be brushed across a surfacecarrying an electrostatic image. The electrostatic attraction of thetoner for the image overcomes the triboelectrically created attractionbetween toner and ferromagnetic particles and the image is developed.Areas of the image exerting less attractive force on the toner than isexerted by the carrier are cleaned of toner as they are brushed. This iscommonly aided by application of an electrical bias to the carrierthrough the applicating surface of the brush. Magnetic brushes also havebeen designed to give either fringe or solid area development byadjusting the conductivity of the carrier. They can also be made to toneareas of less charge and clean areas of greater charge giving what isknown in the art as a reverse development.

Commercial applications of magnetic brush development have been quiteextensive. However, such applications generally have been concentratedin the area of development of electrostatic images formed on anonreusable insulating surface, such as zinc oxide coated paper. Thatis, the toner image is fixed to the insulating surface carrying theelectrostatic image rather than being transferred prior to fixing.

In applying magnetic brush development to systems in which the surfaceto be toned is to be reused after utilization of the toner pattern,certain problems are accentuated. For example, repeated rubbing by thefairly coarse bristles of the magnetic brush has a tendency both toscratch the insulating surface and to create a toner scum on it. Theseproblems are particularly severe when the insulating surface isphotoconductive, as in xerography, because of difficulties in properimage formation on a scratched or scummed photoconductive surface.

Additionally, a powder transfer process greatly increases imagedegradation caused by carrier carryout. More specifically, small carrierparticles are commonly picked up by the insulating surface in the toningoperation and have the effect at toner transfer of holding a large areaof the transfer surface away from the photosensitive surface therebyinhibiting powder transfer. This problem is much reduced if no transferof toner is included in the process, as is the case when the powder isfixed to the toned surface.

SUMMARY OF THE INVENTION It is an object of this invention toeffectively tone an electrostatic image.

It is another object of this invention to minimize surface scratchingand scumming resulting from magnetic brush development of a reusableinsulating surface.

It is another object of this invention to improve triboelectricdeveloper agitation in magnetic brush development thereby providing goodquality development with low toner concentrations.

It is another object of this invention to reduce carrier carryout inmagnetic brush development.

It is another object of this invention to effectively tone areas desiredto be solid black.

It is another object of this invention to provide a mag netic brushdeveloping apparatus with a soft, Well-mixed toning medium and reducedcarrier carryout, which is particularly adapted for use with a reusablephotoconductive surface.

According to the invention, scratching and scumming are reduced by adeveloper feed and discharge arrangement which provide a substantialdeveloper accumulation in a cavity accessible to the image-carryingsurface. This accumulation gives a soft, less abrasive toning mediumwhich, in turn, gives effective toning, including toning of solid areas.

It is a feature of a preferred form of this invention that the feed anddischarge arrangement is so constructed to gently tumble the developeraccumulated in the cavity, giving thorough mixing and triboelectriccharging, and thereby providing effective toning at remarkably low tonerconcentrations.

Although the accumulation of toner can be created and agitated with anumber of forms of apparatus within the concept of this invention, it isa preferred feature of this invention to position two cylindricalmagnetic brushes so that portions of their outer surfaces form a cavitywith the insulating surface to be toned. One brush (herein called thefeed brush) is rotated rapidly to bring developer into the cavity andbarely contacts the image-bearing surface which is moving in theopposite direction. The other brush (herein called the discharge brush)is rotated at a slower speed in the same direction. It picks updeveloper from the feed brush and brushes it lightly across theinsulating surface while carrying the developer out of the cavity.Because of the position and rates of rotation of the two brushes, theform of the magnetic fields created by the brushes, and/ or the relativedirection of motion of the image-bearing surface, there is a build up ofdeveloper in the cavity. This accumulation of developer is in the formof a rollback from the discharge brush, although it may fill the cavity.Because this developer is not as closely associated with the magneticfields of the brushes as developer actually being carried by thebrushes, it is softer and not as bristly or coarse, giving less abrasionand scumming of the insulating surface. The motion of the brushes andthe image-bearing surface causes developer to tumble gently, givingthorough and constant mixing of toner and carrier and good triboelectriccharging.

With the lessening of the influence of the magnetic field, there may bea tendency toward more carrier carryout on the insulating surface. It isa feature of this invention to reduce this carrier or iron carryout bycareful placement of a strong magnet in the feed brush so that itcreates a substantial magnetic attractive force directly in the rollbackarea.

It is also a feature to further reduce carryout, by the feed brushitself which touches the insulating surface so lightly or not at allthat it does very little toning, but is close enough to scavenge awayfrom the surface, carrier that may have adhered thereto despite theaction of the strong magnet.

It is also a feature of this invention to do further carrier scavengingwith an auxiliary magnetic scavenging structure more fully describedbelow.

Other objects and inventive features will appear from the more specificdescription of the preferred embodiments of the invention below.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectionalview of a magnetic brush assembly constructed according to the invention.

FIG. 2 is a fragmentary top view showing the drive train of the magneticbrush assembly of FIG. 1.

FIG. 3 is a schematic cross-sectional view of an alternative magneticbrush assembly constructed according to the invention.

FIG. 4 is a schematic cross-sectional view of an alternative magnetarrangement for the magnetic brush assembly of FIG. 3.

4 DESCRIPTION OF THE PREFERRED EMBODIMENTS Accordinng to FIGS. 1 and 2,an electrostatic charge pattern carried on an insulating surface 2 of aweb 1 driven by drive roller 53 and having a conductive backing 3 is tobe toned. The surface 2 is moved past a discharge magnetic brush 4 and afeed magnetic brush 5 mounted in a developer trough 11 or otherreservoir means. The brushes can be constructed individually accordingto a variety of designs known in the prior art. According to FIG. 1, apreferred design for this application includes a stationary core 10 ofnon-magnetic material around which is mounted a stationary magnetic polepiece 25 that may be made of soft steel or other magnetic material.Mounted around part of the circumference of the pole piece 25 is aseries of permanent magnets 6, for example, rubber-bonded barium ferritemagnetic strips or poles. Concentric with the arrangement of theseelements and on the outside thereof are rotatable, preferablysurfaceroughened, non-magnetic rollers 7 and 8. Each brush isconstructed so that, as the rollers 7 and 8 rotate, developer particlesare held on its surface and moved with the roller while in the field ofthe magnetic strips. The magnetic strips 6 are arranged so that asection of each roller is not enough of the influence of the magneticfield to hold the developer on the roller.

As shown in FIG. 1, the two magnetic brushes 4 and 5 are arranged sothat the feed brush 5 feeds developer to the discharge brush 4. If aclose doctor blade is not used with the discharge brush to define ashort bristle size, a rollback 9 of developer will occur at a point justprior to contact of the discharge brush with the insulating surface.This rollback can be enhanced by several features shown in the drawing.First, the brushes are rotated so that they move in a direction oppositeto the movement of the insulating surface 2 at the points of contacttherewith. Second, the feed brush 5 is rotated at a faster rate than thedischarge brush 4. Third, the feed brush is spaced farther from theinsulating surface 2 than the discharge brush. Either of the second orthird features, if accentuated, are sufiicient alone to create anaccumulation of developer in cavity 15 and, hence, a usable rollback.However, a preferred form of the invention uses all three features incombination to create a rollback of developer of substantial size, whichmay substantially fill the cavity 15 between the brushes and theinsulating surface.

With a substantial accumulation of developer in the rollback cavity 15,this arrangement does most of its effective toning in the softerrollback area 9 with very little toning done in area 12 and 13 ofstiffer bristle. The developer in the rollback which contacts thesurface 2 is not held as firmly by the magnetic fields of either brushas the developer in bristle formation on the rollers and is thereforeless abrasive to the surface 2.. In addition, it is constantly beinggently tumbled, giving good triboelectric charging and effective mixing.

Obviously, developer cannot build up in the rollback cavityindefinitely. Excess toner fed into the brush after the rollback isbuilt up overflows at the ends of the cylinders and falls ito thereservoir 11 to be reused.

Ferromagnetic carrier particles in the rollback area have a tendency tobe occasionally carried away by the insulating surface. This can bereduced by aiming at strong magnet 19 in the second brush at therollback 9. The strength of this magnet is preferably worked outcmpirically for the softness desired in the rollback and the carryoutpermissible. Further, the feed brush, which lightly contacts theinsulating surface after the surface passes the rollback area, iseffective to remove some of those carrier particles actually picked upby the electrostatic image in toning.

A scavening device 30 can also be used. It includes a stationary singlestrong magnet 31 placed in close proximity with the insulating surface 2and a rotatable nonmagnetic roller 32 between the magnet and surface 2to carry out ferromagnetic particles attracted toward the magnet.

Many alternative mechanisms may be used to drive the rollers 7 and 8 atdifferent speeds. One such mechanism is shown in FIG. 2 in which asingle drive gear 29 connected to a source of power, not shown, drives agear connected by a shaft 26 to roller 8 and drives a gear 27, which hasa larger diameter than gear 28, and is connected through a shaft 24 toroller 7. The difference in the diameters of the gears will cause roller8 to be rotated at a faster speed than roller 7. Alternatively, thegears can be made the same size and the rotations per minute of therollers 7 and 8 be made equal, but the second roller 8 be made of largerdiameter than the first roller 7, thereby providing a greater peripheralspeed. Alternatively, the size and speed of rotation of the rollers canbe made equal, but the strength of the magnets and, hence, the size ofthe bristles in the feed brush made greater than in the discharge brush.

Similarly, many alternative means may be used for replenishing tonerused up in the process. For example, as shown in FIG. 1, a tonerdispenser 50 including a hopper 51 allows toner to fall at a regularrate onto a paddle wheel 52 which rotates and feeds toner into thetrough 11. Fresh toner is circulated gradually toward the bottom of thefeed brush by paddle wheels 21 and 22, Where it is picked up by thesecond brush to begin use in the process.

FIGS. 3 and 4 show an alternative design invented by R. A. Drexler, oneof the co-inventors herewith and included in said above-mentionedco-pending application. According to FIGS. 3 and 4, a single magneticbrush 63 is constructed essentially the same as each magnetic brush inFIG. 1 except that a feed portion 68 of the brush contains strongermagnets than a discharge portion 70. Strong feed magnets 61 are attachedto a pole piece 67 on a central core 66, and weaker discharger magnets62 are attached to a pole piece 64. The magnets 61 in the feed portion,being stronger, hold more developer on an outside surface of a rotatingroller 65 than is held by the same roller under the influence of weakermagnets 62 in the discharge portion 70, and a rollback is created in arollback cavity 69. In this embodiment, the rollback cavity 69 isdefined by the rotating roller 65 in the discharge portion of themagnetic brush together with the insulating surface 2, and is open onone side as well as both ends.

In FIG. 4 is shown an alternative design for the apparatus of FIG. 3 inwhich the magnets associated with the feed portion of the magnetic brushare not strips as shown in FIG. 3 but are horseshoe magnets 71 embeddedin core 66.

In addition to producing a softer medium which is less harmful to aphotoconductive or other insulating surface, the broad soft developmentsurface also gives excellent image resolution. The rollback actionprovides a continuous soft mixing of toner and carrier helping tomaintain the proper polarity of charge on the toner at the developmentarea. This continuous mixing combined with the effectiveness of thetoning allows for substantial variations in overall toningconcentrations with toning results that are very close in quality. Ifthe brushes are used as development electrodes, the apparatus will giveespecially uniform blacks in solid image areas.

As an example of the invention, two brushes are constructed as shown inFIG. 1. The rotating cylinders 7 and 8 are made 3 inches in diameter andA inch apart at their closest outside points. The strip magnets 6 haveenough magnetic attraction for iron particles of +80 l20 mesh to producebristles approximately inch long. The axes of cylinders 7 and 8 areoffset with respect to the insulating surface 2 by A inch with theperiphery of feed cylinder 8 approximately 7 inch and discharge cylinder7 approximately /8 inch from insulating surface 2, respectively, attheir closest points. With surface 2 moving at 20 inches a second, feedcylinder 8 is rotated at r.p.m. and discharge cylinder 7 is rotated at40 r.p.m. Excellent toning is obtained at these speeds With biasadjustments as in prior magnetic brushes. As toner concentration isvaried from 3 to 6 percent by weight, no difference in print quality isnoticeable to the naked eye. This remarkable and highly useful featureis believed to be due to the excellent mixing and tribe-electriccharging obtained as the developer tumbles in the rollback cavity 15.Although 65 and 40 r.p.m. appear to be optimum speeds for theseparameters, little difference actually is noticed if the cylinders 8 and7 are rotated at a variety of other speeds, for example, at 40 and 30r.p.m., respectively. The parameters giving best results with anyparticular equipment, developer and type of image are preferably workedout empirically with that equipment, developer and type of image.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effective within the spirit and scope of theinvention as described hereinabove and as defined in the appendedclaims. For example, although the invention has been described withreference to magnetic brush means for transporting toner into and out ofa rollback cavity, it can be seen by such description that other meansof creating a soft tumbling development medium can be used within thespirit of the invention. For example, fiber brushes, rollers withridges, etc., could also be used to create a similar effect.

We claim:

1. A method of developing an electrostatic charge pattern on aninsulating surface, said method comprising:

(a) moving the insulating surface in a first direction across adevelopment zone;

(b) magnetically transporting a developer mixture of iron particles andtoner particles from a reservoir to the development zone at a firsttransport rate and from a direction opposite to said first direction;and

(c) magnetically transporting the developer mixture out of thedevelopment zone at a second transport rate which is sufficiently lessthan said first transport rate to provide a substantial accumulation ofdeveloper mixture in the development zone, which accumulation issubjected to a lesser magnetic attraction than the magnetic attractionduring its transport;

whereby the movement of insulating surface causes the accumulation toroll back and tumble in the development zone causing an increased degreeof contact between the particles of the developer mixture and theinsulating surface.

References Cited UNITED STATES PATENTS 3,318,284 5/1967 Hojo et all17-17.5 3,402,698 9/1968 Kojima et al. 1l7-l7.5 3,133,834 5/1964 Sowiak118-637 3,145,122 8/1964 Streich 118-637 3,219,014 11/1965 Mott et a1.118637 3,256,855 6/1966 Olip'hant 118-637 3,355,288 11/1967 Matkanll717.5 3,543,720 12/1970 Drexler et a1 118-637 FOREIGN PATENTS1,218,287 6/1966 Germany 118-637 17,228 8/1965 Japan 118-637 OTHERREFERENCES Medley: Development of Electrostatic Images, IBM TechncalBulletin, vol. 2, No. 2, p. 485, August 1959.

WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner

